I just flew from Santo Domingo to Madrid on IB6500. I bought the ticket 6 months ago. I chose Premium Class and l reserved a seat.
A month later Iberia informed me that there would be No Premium Class on that flight so they moved me to Economy. They would not give me a refund until after the flight.
So here�s the weird thing: when l boarded l saw that there was indeed Premium on the plane. In fact l walked right past the seat that l had reserved, now occupied by someone else. Maybe they could sell it a higher price to him? I don�t know. Iberia says nothing. Is this ok?

Presumably you are referring to Premium Economy and not just a better seat in Economy class.

Under EC261, IB needs to refund you 75% of the fare you paid for Premium Economy. This is totally black and white. If you don't receive that 75% you can take IB to small claims court or AESA. https://europa.eu/youreurope/citizen...deAndDowngrade

In the future, keep in mind that you can ask to move to a different date with a PE cabin or to keep calling back to see whether the aircraft assignment has changed again.

Which routes does IB use their PEless 332s and exHainan 359s on?

Likely that the aircraft was swapped to a 332, then swapped again to 333, potentially at the last minute.

Most likely the case

That certainly would make sense but that is not what happened. The cabincrew told me that they always had Premium Economy on this flight. And I could see that they had been flying with this aircraft for as long as Flightradar could show. They just sold the seat several times.

And matter the reason they should refund me the difference between Premium Economy and Economy. Iberia just says its sorry, but they don�t refund anything which I understand they are infamous for.

Crew wouldn't know every single aircraft swap that's happened between the flight being loaded into the schedule and takeoff. They likely don't even pay attention to that until the pre-departure briefing. And IB does swap aircraft around quite a bit (or at least, throws a mix of aircraft on any given route, though often pre-arranged as such) - the benefit in having a common crew pool.
The SDQ route is a solid mix of 332/333. In January alone, 14 flights were 332, 1 flight was 359, and 30 / 31 Jan are expected to be 332. December, similar picture - 18 flights on 332, 1 359, the rest 333.

They should not refund the difference between the cabins - they should refund 75% of that segment, net of taxes. Search FlyerTalk for the Mennens formula. Easiest to ask IB for the refund calculation directly, but do your own math and see if it's right. And if they don't budge, log a claim with AESA.

Is this true despite the change happening much before the two week deadline?

Yes. The deadline is only for compensation for delayed/cancelled flights. And to note - this is a 75% refund, not compensation, to be pedantic.